1. Field of the Invention
The present invention is generally related to direct mount heat sinks for heat generating devices used in electronic applications and in particular to heat sinks containing Group VIB metals from the periodic table of the elements.
2. Description of the Prior Art
Refractory metals, such as molybdenum, have long been used as direct mount heat sinks for heat generating devices used in electronics. Its high thermal conductivity, on the order of 140 W/M° K, in some cases provide adequate thermal conductivity and a close thermal expansion match (TCE of 5.1 ppm/° C.) to materials like silicon.
Such materials often have poor solderability, which can be improved by applying a thin layer of Ni on to one or both surfaces. Also, layers of Cu are often deposited as a thin layer, such as by cladding, spraying, etc., on to the surface in order to alter the thermal expansion properties to match other devices, such as for example GaAs containing devices (TCE of 6.5 ppm//° C.). Such materials having thin Cu layers (laminates) often have unpredictable expansion characteristics during thermal cycling due to uneven distribution of the material in the surface layer or from layer to layer.
In some instances, metal matrix composites such as powder metal matrices of tungsten or molybdenum infiltrated with copper have provided improved thermal characteristics to meet the requirements of either a closer TCE match or higher thermal conductivity. A powder matrix of this type is limited as to the amounts of higher thermal conductivity materials which can be added without creating thermal expansions that are too high.
Such composite matrix materials are inconsistent in structure and do not perform as predicted by the law of mixtures as the high thermal conductivity copper matrix is often not open enough to conduct heat in an unrestricted fashion (i.e., narrow paths, flow blocked by touching refractory metal particles, etc.). Thus, some of the heat must be transferred through the lower thermally conductive refractory metal matrix. Additionally, low levels of porosity exist in the matrix structure restricting thermal flow.
In many cases laminate or matrix systems of Mo—Cu or W—Cu are made into thin structures by way of standard metallurgical procedures such by sawing, cutting, rolling, grinding, and/or lapping, which induce stresses in the material that cannot be completely relieved. The stresses cause warping of the material at thin gauges when exposed to elevated temperature soldering processes.
Materials commonly used for electronic packaging include Al-Graphite, Cu-Graphite, CuMoCu laminate, CuMoCu laminate with a MoCu powdered metal core, W—Cu metal matrix composites, Mo—Cu metal matrix composites, SILVAR® (available from Engineered Materials Solutions, Inc., Attleboro, Mass.), Al—Si metal matrix composites, Al—SiC metal matrix composites, and Cu—SiC metal matrix composites.
U.S. Pat. No. 4,996,115 discloses a composite structure and a method of producing said composite structure from a combination of copper and a low coefficient of thermal expansion nickel-iron alloy where the copper clads the nickel-iron sheet and is interposed through the central nickel-iron sheet in such a fashion as to provide a substantially isotropic heat transfer path. However, the rolling process used to forge the copper-clad sheet results in non-uniform elongated holes that can result in non-uniform heat transfer and dissipation. Additionally, the large hole size, 40-60 mil, is generally not appropriate for electronic applications.
U.S. Pat. No. 5,011,655 discloses a method of manufacturing a thin metallic body composite structure. An inner layer of a first metal is cleaned to remove oxides and promote metallurgical bonding. The inner layer has a plurality of penetrating holes piercing the thickness of the inner layer. The penetrating holes are filled with metal powder of a second metal. Two outer layers of the second metal are placed on opposite sides of the cleaned and filled inner layer to form a sandwich structure. The sandwich structure is heated to a temperature at which recrystallization will occur in a non-oxidizing atmosphere. The sandwich structure is then hot worked to reduce thickness of the sandwich structure forming the thin metallic body composite structure. Unfortunately, the hot working procedure used to forge the composite structure can result in non-uniform elongated holes that can result in non-uniform heat transfer and dissipation. Additionally, although the composite structure is targeted for use in electronic applications, the large hole size, 40-62 mil, is generally not optimal for such applications. Further, porosity of the powder used to fill the holes detracts from its ability to conduct heat.
U.S. Pat. No. 5,156,923 discloses a metal composite containing layers of copper and Invar, which are cold pressure rolled with reduction in thickness to be metallurgically bonded together in interleaved relation, and strips of the bonded materials are cold pressure rolled together a plurality of times with reduction in thickness to be metallurgically bonded together. The resulting metal composite breaks up the layers of Invar to distribute portions of the Invar material in a copper matrix, which limit thermal expansion of the composite. However, the composite has a limited ability to dissipate heat vertically or in the z-axis direction.
A particular limitation on the use of composite structures is that they are typically porous and cannot be used, for example, in applications where gas or air leakage needs to be prevented, as for example in satellite applications, especially in structures less than 20 mil thick.
U.S. Pat. No. 6,555,762 discloses a high density, electronic package having a conductive composition for filling vias or through holes to make vertical or Z-connects from a dielectric layer to adjacent electrical circuits. The through holes may be plated or non-plated prior to filling.
The above-described matrix materials are also very difficult to obtain in the desired thickness range of less than 20 mils without considerable processing which builds up stress in the refractory metal matrices, which cannot be relieved by thermal processing because of the low melting point of the high thermally conductive infiltrant.
Obtaining a thin material is extremely important because of the thermal relationship:R=Const. L/KA where R is thermal resistance, L is the distance the heat flows or the thickness of the spreader, K is the thermal conductivity of the thermal spreader and A is area. The lower the thermal resistance, the better the performance as a heat sink, which is impacted as follows:                Shorter distance for heat flow and thinner spreaders provide better performance.        Higher thermal conductivity results in lower thermal resistance.        The greater the area that the heat can be spread over, the lower the thermal resistance.        
Luedtke, Thermal Management Materials for High-Performance Applications, Advanced Engineering Materials, 6, No. 3 (2004), pp. 142-144, discusses copper coated molybdenum and copper coated molybdenum-copper matrix materials as heat spreaders. However, these materials only provide effective thermal conductivities between 190 and 250 W/M° K.
Generally, the prior art discloses materials, such as Ni—Fe alloys with vias designed to improve z-axis thermal conductivity, that approach the properties of refractory metals. However, current needs, especially in the electronics industry, are for materials that surpass the properties of refractory metals. For example, materials with more, but smaller uniform holes that can be filled without stressing the material and are able to dissipate heat in all directions with superior properties to currently available refractory metal systems as well as other systems.
Thus, there is a need in the art for heat sink materials useful with heat generating electronic components that are sufficiently thin and can adequately conduct, remove and dissipate the generated heat in all directions while maintaining dimensional stability.